This is really too simplistic.
You're assuming that because some people aren't satisfied with subpar LC play, that we some how prefer Henne or a second rate QB. That's just not the case. I don't think I've heard anyone prefer that. The fact is, that our concern for the LC position to include JM's underperformance .... or to put it a different way .... his lack of progress from last year, is separate of the excitement I have for a promising young QB I have faith in for the first time since Marino.
No one that I have seen is arguing that QB isn't the most important position, but we are, as someone else put it, pointing out that ALL positions are important. And like it or not, a sub-decent LC is a problem.
Again, you'll probably notice that most people voicing their concern, including myself, are not looking to throw JM under the bus. He needs some time develop his pass protection, I don't think that's arguable. The problem is .... right now, he doesn't look like he has, and since as you stated QB is the most important position, the QB's health and well being is problematic with a turnstile at LC. So by your own (and others) argument, even if a LC isn't as important a player, he better be at least a RELIABLE player.
I don't have a solution at this point. I'm obviously just a fan, but I believe it's a legitimate issue.